1. Field of the Invention
The present invention relates to tuned mass damping devices and more particularly to such dampers which may find use in reducing the periodic motion of elongated structures such as booms. The invention may have particular utility with booms mounted on satellites to hold measuring equipment the accuracy of which may be reduced due to the sway of the boom resulting from disturbances such as thermal distortion shock caused by, for example, transient thermal distortions of solar panels.
2. Description of the Prior Art
In the prior art, tuned mass dampers for reducing sway are known. Such dampers usually contain a mass mounted for movement in a container of fluid or a magnetic field and positioned by a spring. By proper selection of the mass and spring, the mass will have the same natural frequency as the boom, or other device to which the damper is mounted, so that when the boom experiences shock and begins to sway in a direction, the mass begins to vibrate or oscillate in the same direction and at substantially the same frequency. However, since the boom is an input to the damper, the damper vibrates 180 degrees out of phase with the boom, which motion tends to cancel the boom motion. Since the boom is now vibrating at an off-resonant frequency and the damper has absorbed a substantial portion of its energy, the boom displacement is much smaller and is effectively damped out by the fluid or by the magnet in the damper. Such dampers are satisfactory for high frequency vibrations but because frequency is proportional to the ratio between the square root of the spring constant to the mass, at low frequencies e.g. 1.5 hertz, the mass becomes too large for the spring and cannot be effectively supported. The result is that the mass begins to sway and move in directions other than that required for proper damping.
The present invention overcomes the problems in the prior art by providing a damper with a mass which is constrained to move in the desired direction. By making the mass cylindrical and positioning it within a housing closely adjacent the mass, motion in the fluid container in only the desired direction is permitted. The fluid may be varied to make the vibration tunable and, in fact, the present invention provides for tunable damping without having to change the fluid. The tuning of the damping is accomplished by providing a pair of bellows with changeable internal springs therein to change the volumetric stiffness of the bellows and thus provide different characteristics to the fluid expansion and contraction in the chambers surrounding the mass. Thus, in testing the damping characteristics for a particular use, only the springs internal to the bellows need be changed for fine tuning. In the event that the sliding friction between the mass and the adjacent container housing is too large, a nearly frictionless motion is provided by using a linear bearing with, for example, circulating balls. A specific improvement to the ball bearing mounting is shown in the present invention by the use of a plurality of linear troughs in the mass each of which entraps a single ball so that there is no sliding friction between the mass and the walls or between adjacent balls.